In the art of heating, cooling and otherwise “conditioning” the air of buildings, “two-pipe” hydronic systems are known. In such systems either hot or cold water, typically from a central location, is circulated about a piping loop. Fluidly attached to said loop, and disposed in zones of the building the air of which is to be conditioned, are a plurality of water-to-air heat exchangers. By regulating the amount of water and/or air which is passed through a particular water-to-air heat exchanger the air of the associated zone may be warmed or alternatively cooled a desired amount. The invention disclosed and claimed herein relates to a thermal reservoir means whereby water stored in said thermal reservoir at one temperature (for instance cold) may be used to reversibly displace water circulating about the piping/heat exchangers at a different temperature (for instance hot) circulating about said piping/heat exchangers, thereby permitting rapid switching of a two-pipe hydronic air-conditioning system between heating and cooling modes while conserving the energy content of both the stored and circulating water.